In ion projection lithographic systems, the structure of a mask can be imaged or reproduced in a substrate (e.g. a wafer of semiconductive material), by means of an ion beam. The imaging is effected by ion optics disposed downstream of the mask and comprised of a combination of ion optical lenses. In the case of electrostatic lenses, the imaging characteristics, like, focal length and principal planes, are defined by the so-called potential voltage ratio.
The voltage ratio R is understood, for two-electrode lenses (so-called immersion lenses) with the potentials V.sub.o, and V1 for the two lenses, as R=E.sub.o /E.sub.i where Ei is the input energy of the ions and E.sub.o is the output energy of the ions. For the case in which E.sub.i =-V.sub.o.e (where e is the elemental charge unit or electron charge), R=V1/V.sub.o.
For a three electrode lens operated as a so-called EINZEL lens, the three electrode potentials in succession are V.sub.o, V.sub.1, V.sub.o, for which R=(Ei-(V1-Vo).e/Ei), where again e is the elemental charge. For the case in which Ei=-Vo.e, R=V1/Vo.
By applying the corresponding potentials to the electrodes, the voltage ratio for the desired imaging characteristic can be obtained for predetermined values of the ion energy Ei or Eo. Every deviation of the ion energy from the basic value Ei results in a variation, as noted above, in the potential ratio R and therefore a different image scale for the same point of the mask. That means that the resolution decreases. The greater the energy blurring or spread of the energy spectrum of the ions deriving from the ion source, the poorer is the resolution of the structure formed upon the wafer. The magnetic lenses operate analogously and the settings thereof also are applicable only for a predetermined energy value of the ions.
Another parameter which determines the resolution is the size of the so-called virtual ion source. The location of the virtual ion source is found by projecting the ion trajectories leaving the extraction system, i.e. from the field free space, rearwardly towards their origin and locating the smallest diameter of that bundle. The size of the virtual source is given by that diameter. The smaller the virtual source, the greater is the resolution.
For ion projection lithographic systems, ion sources are required at which the diameter of the so-called virtual ion source is held as small as possible, preferably smaller than or equal to 10 microns. For lithographic systems, duoplasmatron ion sources have been found to be particularly effective because with these sources there is a practically laminar extraction of the ions and, at a diameter of the outlet of the plasma chamber and of the source anode closing this outlet of 200 microns, the virtual ion source can attain a diameter of about 10 microns (see Austrian Patent 386 297).
For the reasons stated, a further critical point in the ion projection lithographic system is the distribution of the energy of the ions over the mask against which the ions impinge. Since the potential ratio can only be set for a single input energy of the ions at the imaging lenses, ions whose input energy deviates from the predetermined energy E.sub.o, result in an unsharp reproduction of the mask structure upon the wafer and thus a reduction in the resolution.
Previously used duoplasmatron ion sources (R. Keller, "A High-Brightness Duoplasmatron Ion Source", in "Ion Implantation: Equipment and Techniques", Springer Series, ed. H. Ryssel, H. Glawischnig) have an anode-bounded plasma chamber in which gas particles are ionized by electrodes and/or ions are produced with the aid of a thermonic generator and then passed via two further electrodes of which one (the so-called suppressor electrode) is at a lower potential than the anode and a second electrode (the so-called extraction electrode) is at a higher potential than the suppressor electrode but which is still significantly less than that of the anode. The ions are thus accelerated to their final energy by the potential difference between anode and extraction electrode and the ions thus are passed from the plasma chamber with an energy which is the sum of the starting energy from the plasma chamber and the acceleration energy applied in the electrode assembly. The suppressor electrode serves to suppress the secondary electrons which arise at the extraction electrode and pass counter to the ion current.
Because of the statistical energy distribution within the plasma corresponding to the plasma temperature, the heretofore used duoplasmatron ion sources are characterized by a so-called intrinsic energy unsharpness of about .+-.5 eV with respect to the mean value given by the extraction potential.
An additional contribution to energy distribution of the ions impinging upon the mask results, in prior duoplasmatron sources, from ions which are first formed downstream of the anode by impact ionization of neutral gas particles. Such impingement ionization occurs primarily in the region between the anode and the suppressor anode because of the comparatively high ion current density predominating in this region. Since these ions travel only over a part of the acceleration stretch, their energy is significantly less than that of the ions emerging from the plasma chamber. Since this effect may produce a relatively high number of ions with relatively low energy as compared with the predetermined energy E, the contribution of these ions to the energy distribution of the ions impinging at the mask may be significantly higher than that of the intrinsic energy spread.